Nelibur - Nelibur is message based web service framework on the pure WCF. Nelibur simplifies creating high-performance and message based web services and you certainly have all the power of the WCF.

WebAPI Contrib - Collection of open source projects to help improve your work with ASP.NET Web API

Application Frameworks

ASP.NET Boilerplate - A starting point for new modern ASP.NET MVC web applications with best practices and most popular tools.

Orleans - Orleans is a framework that provides a straight-forward approach to building distributed high-scale computing applications, without the need to learn and apply complex concurrency or other scaling patterns

CoreFX - The corefx repo contains the library implementation (called "CoreFX") for .NET Core. It includes System.Collections, System.IO, System.Xml and many other components. It builds and runs on Windows. You can 'watch' the repo to see Linux and Mac support being added over the next few months.

Power Args - PowerArgs converts command line arguments into .NET objects that are easy to program against. It also provides a ton of optional capabilities such as argument validation, auto generated usage, tab completion, and plenty of extensibility

CLR

CoreCLR - The coreclr repo contains the complete runtime implementation (called "CoreCLR") for .NET Core. It includes RyuJIT, the .NET GC, native interop and many other components. It builds and runs on Windows. You can 'watch' the repo to see Linux and Mac support being added over the next few months.

CMS

Umbraco - Umbraco is a free open source Content Management System built on the ASP.NET platform

Nemerle - Nemerle is a high-level statically-typed programming language for the .NET platform. It offers functional, object-oriented and imperative features. It has a simple C#-like syntax and a powerful meta-programming system. http://nemerle.org

Netjs - .NET to TypeScript and JavaScript compiler. Portable Class Libraries work great for this. You can even pass EXEs.

Roslyn - The .NET Compiler Platform ("Roslyn") provides open-source C# and Visual Basic compilers with rich code analysis APIs. It enables building code analysis tools with the same APIs that are used by Visual Studio.

Compression

SharpCompress - SharpCompress is a compression library for .NET/Mono/Silverlight/WP7 that can unrar, un7zip, unzip, untar unbzip2 and ungzip with forward-only reading and file random access APIs. Write support for zip/tar/bzip2/gzip are implemented

Deployment

DirectX

SharpDX - SharpDX is an open-source project delivering the full DirectX API for .Net on all Windows platforms, allowing the development of high performance game, 2D and 3D graphics rendering as well as realtime sound application.

Distributed Computing

Project Orleans - Orleans is a framework that provides a straight-forward approach to building distributed high-scale computing applications, without the need to learn and apply complex concurrency or other scaling patterns. It was created by Microsoft Research.

Akka.net - Akka.NET is a port of the popular Java/Scala framework Akka to .NET. This is a community driven port and is not affiliated with Typesafe who makes the original Java/Scala version.

GUI

Callisto - A control toolkit for Windows 8 XAML applications. Contains some UI controls to make it easier to create Windows UI style apps for the Windows Store in accordance with Windows UI guidelines.

ObjectListView - ObjectListView is a C# wrapper around a .NET ListView. It makes the ListView much easier to use and teaches it some new tricks

CSScript - CS-Script is a CLR based scripting system which usesC# as a programming language. CS-Script currently targets Microsoft implementation of CLR (.NET 2.0/3.0/3.5/4.0/4.5) with full support on Mono. Comes with many additional features, such as script hosting.

LINQPad - a C#/VB/F# scratchpad that instantly executes any expression, statement block or program with rich output formatting and a wealth of features. Also lets you interactively query databases in LINQ. [$]

ScriptCS - Write C# apps with a text editor, nuget and the power of Roslyn!

MVVM

Caliburn.Micro - A small, yet powerful framework, designed for building applications across all XAML platforms. Its strong support for MV* patterns will enable you to build your solution quickly, without the need to sacrifice code quality or testability.

MVVM Light Toolkit - The main purpose of the toolkit is to accelerate the creation and development of MVVM applications in WPF, Silverlight, Windows Store (RT) and for Windows Phone

UpdateControls - Update Controls does not require that you implement INotifyPropertyChanged or declare a DependencyProperty. It connects controls directly to CLR properties. This makes it perfect for the Model/View/ViewModel pattern.

Okra App Framework - An app centric MVVM framework for Windows 8.1 built with dependency injection in mind, including a full set of Visual Studio MVVM templates.

WPF Application Framework (WAF) - A lightweight Framework that helps you to create well structured WPF Applications. It supports you in applying a Layered Architecture and the Model-View-ViewModel pattern.

Web Servers

XSP - Mono's ASP.NET hosting server. This module includes an Apache Module, a FastCGI module that can be hooked to other web servers as well as a standalone server used for testing (similar to Microsoft's Cassini)